1. Field of the Invention
The present invention relates to a solid-state image pickup apparatus including photosensitive cells for electrophotographic transduction arranged in a so-called honeycomb arrangement and a solid-state image sensor included in the image pickup apparatus for generating signal charges with the photosensitive cells.
2. Description of the Background Art
Generally, a solid-state image pickup apparatus is apt to generate moiré and other false signals. Japanese patent laid-open publication No. 136391/1998, for example, discloses a solid-state image pickup apparatus configured to increase the quantity of incident light with densely arranged pixels, thereby promoting efficient receipt of the light. The apparatus taught in this document provides a new structure that optimizes spatial sampling of an image and is referred to as a honeycomb pixel arrangement.
In the honeycomb pixel arrangement, assuming that the distance between nearby pixels on the same row or the same column is a pitch, then pixels around a given pixel each are shifted from the given pixel by half a pitch in the direction of row and/or the direction of column. In a CCD (Charge-Coupled Device) type of solid-state image sensor using the honeycomb pixel arrangement, vertical transfer registers constitute vertical transfer paths extending zigzag in such a manner as to skirt round the pixels. Color filter segments are assigned to, among the pixels, actual pixels that actually exist. The actual pixels, or photosensitive cells, photoelectrically transduce light incident thereto via the color filter segments to generate signal charges having color attributes. The signal charges are then sequentially routed through the vertical transfer registers and horizontal transfer registers, which constitute a horizontal transfer path perpendicular to the vertical transfer path, to an output amplifier. The output amplifier performs Q/V conversion for outputting voltage signals in the form of analog signals.
Subsequently, the analog signals are subjected to signal processing. First, a correlation between pixel data is determined with consideration given to the colors of actual pixels. More specifically, pixel data of the same color and closely correlated to each other are used to estimate pixel data at a virtual pixel, as distinguished from the actual pixels, by calculation and pixel data at actual pixels of different colors. Subsequently, one of such pixel data closer in correlation than the other pixel data is interpolated in the virtual pixel. Such interpolation successfully reduces false signals. Further, the pixel data are successfully broadened in frequency band, enhancing resolution.
It has also been proposed to improve the honeycomb pixel arrangement for further enhancing the resolution of an image and broadening the dynamic range of image signals generated. In accordance with a specific conventional scheme directed toward this object, each of the photosensitive cells has its photosensitive area segmented into a major region and a subregion smaller in area than the former, so that signal charges are readout from the two regions independently of each other. This scheme broadens the dynamic range on the basis of a difference in sensitivity between the main and subregions. In a usual reading mode, the signal charges of the main and subregions are mixed together and read out in the conventional manner.
In the main and subregion scheme stated above, the two regions of the individual photosensitive cell are positioned in a single optical opening. At the light incidence side of the optical opening, a single microlens is positioned so as to focus the incident light onto the photosensitive cell via the optical opening. The spatial information resultant from the focusing is identical with spatial information available with a photosensitive cell having a single region. Because the main and subregions are different in sensitivity from each other due to the difference in area, signals output from the two regions are different from each other for the same spatial information. By combining the signals derived from the two regions by signal processing, it is possible to provide image signals with a broader dynamic range unachievable with the conventional, signal charge-mixing scheme.
Further, in the solid-state image pickup apparatus, a G (green) filter segment is positioned at the light incidence side corresponding to the subregion, so that the subregion outputs a signal close to, among spectral components derived from incident light, a luminance component. By using the luminance component, it is possible to enhance the resolution of an image.
However, optical information conveyed through a microlens is spatially dealt with as identical spatial information, as stated earlier. Consequently, despite that the photosensitive cells are densely arranged, the spatial resolution of the honeycomb pixel arrangement has an upper limit dependent upon the pixel pitch. It follows that an image with resolution higher than the upper limit of spatial resolution would not be rendered without suffering from false signals.